Squeak! Ancient Helium Escaping from Yellowstone

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The giant magma blob beneath Yellowstone National Park unleashed
tons of ancient helium gas when it torched North America,
according to a new study.

"The amount of crustal helium coming out is way more than anyone
would have expected," said Jacob Lowenstern, lead study author
and scientist-in-charge at the U.S. Geological Survey's
Yellowstone Volcano Observatory. The findings appear in
today's (Feb. 19) issue of the journal Nature.

Yellowstone National Park's famous geysers burble within the
remains of a supervolcano that first exploded 2.1 million years
ago. Both the volcano and the geysers owe their existence to a
hotspot, a massive plume of molten rock rising from within
Earth's mantle toward the surface. [ Infographic:
The Geology of Yellowstone ]

However, before western
North America trundled over the hotspot, Yellowstone's future
birthplace went undisturbed for more than 2 billion years, the
study authors think. This gave the continental rocks plenty of
time to build up big helium-gas stores via the decay of
radioactive elements in the crust. (Most helium created on Earth
comes from uranium and thorium.) The helium was probably trapped
both in rocks and in fluids such as groundwater.

Because all was quiet in this corner where the West meets the
Plains, no tectonic grinding freed the helium
via fractures such as faults. Only when the hotspot drilled
through the crust a couple of million years ago could the helium
finally escape. The rising magma heated and cracked the crust,
releasing the gas, and provided a path to the surface via
churning fluids and molten rock.

Pop, hiss, squeak!

Lowenstern compared the process to a faucet. "It was like a
spigot that was turned off for a really long time, not allowing
the helium to migrate through the crust, and when the hotspot
came to Yellowstone, it turned the spigot back on," he said.

Lowenstern and his co-authors discovered the strange quantities
of helium a few years ago, while tracking gases at Yellowstone.
The gases — such as carbon dioxide, sulfur and helium — help
reveal what the magma beneath the surface is doing.

The researchers measured two helium isotopes, which are atoms of
the same element with different numbers of neutrons. Helium-3 has
one neutron, and helium-4 has two.

The two helium isotopes come from different sources. Helium-3
created on Earth forms primarily in the mantle, the thick, hot
layer between the crust and the core. Helium-4 is produced
primarily in the crust from the decay of radioactive elements.
The proportions of helium-3 and helium-4 in a gas sample reveal
how much helium comes from the mantle and how much comes from the
crust.

Yellowstone's helium-gas emissions were already known for their
high helium-3 content, a sign of the massive mantle hotspot
beneath the region. But the proportion of helium-4 in the gases
escaping from the crust is also much higher than expected, the
study reports.

"It's a little bit surprising," Lowenstern told Live Science's
Our Amazing Planet. "Not only is there this really impressive
mantle
source, but there's also this crustal source, too. It was
definitely exciting to see this old signature."

Small but strong

Helium is just a tiny fraction of the gases escaping Yellowstone
each day. The park produces about 350 lbs. (160 kilograms) of
helium gas each day, but 44 million to 110 million lbs. (20
million to 50 million kg) of carbon dioxide daily, Lowenstern
said.

Even so, the quantity of helium-4 in Yellowstone's gas emissions
is hundreds to thousands of times greater than it should be — a
sign that the crust is releasing its ancient stores of the rare
isotope, the researchers said.

The findings add to growing evidence that Earth's crust can cache
gases and fluids for billions of years, providing a window into
the planet's early past. In 2013, researchers reported finding a
pocket of
2.6-billion-year-old water in a deep mine in Ontario, Canada.

"This work is another important example of very ancient fluid
accumulations in the crust," said Greg Holland, a geochemist at
Lancaster University in the United Kingdom who was not involved
in the study. "It adds to the evidence that the crust has the
important ability to store [gases] over geologically significant
timescales, with implications for evolution of subsurface life
and also how the mantle-crust-atmosphere system operates over
geological time."

Editor's note: This story was updated Feb.
20 to correct the proportion of helium-3 to helium-4 on
Earth.